<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            width: 600px;
            height: 100px;
            border: 1px solid #000;
            display: flex;
        }

        .a1 {
            width: 100px;
            border: 1px solid #000;
            /* 禁止收缩 */
            flex-shrink: 0;
        }

        .a2 {
            width: 600px;
            border: 1px solid #000;
            flex-shrink: 0;
        }
    </style>
</head>

<body>
    <div class="a">
        <div class="a1"></div>
        <div class="a2"></div>
    </div>

</body>

</html>